1. Field of the Invention
The present invention relates to a method and a device for driving a directional speaker that vibrates a diaphragm with an electric signal supplied from an external source to generate sound waves in the ultrasonic wave range. More particularly, the present invention relates to a method and a device for driving a directional speaker that generates an audible sound field using the narrow-directional characteristics that are characteristics of ultrasonic waves.
2. Description of the Prior Art
An ultrasonic speaker, which generates an audible field using the narrow-directional characteristics that are characteristics of ultrasonic waves, is known as a directional speaker. An ultrasonic speaker, mounted on an electronic device, utilizes the narrow-directional characteristics to give an effect that only the user can hear the sound.
One known ultrasonic speaker has a configuration in which many ultrasonic speakers are arranged in an array form to give directional characteristics through a parametric effect (see Patent Document 1).
The overview of this directional speaker will be described with reference to FIG. 23 and FIG. 24. FIG. 23 is a top plan view showing the configuration of a directional speaker, and FIG. 24 is a cross section diagram showing the configuration of an ultrasonic speaker of the directional speaker.
As shown in FIG. 23, a directional speaker 109 is an ultrasonic speaker array configured by arranging a plurality of ultrasonic speakers 100, each generating many ultrasonic waves, in an array form on a printed circuit board 108. Inputting ultrasonic signals, amplitude demodulated with audible signals, into this ultrasonic speaker array generates a directional sound field.
Using the modulated signal generated by amplitude modulating the ultrasonic signal that is the carrier wave, the directional speaker 109 shown in this figure drives the ultrasonic speaker 100 with an audible sound and outputs ultrasonic waves. The ultrasonic waves, which are output from the ultrasonic speaker 100, generate secondary audible sound waves of audible sounds through the non-linear phenomenon of ultrasonic waves while traveling through air and give a parametric effect.
As shown in FIG. 24, each of the ultrasonic speakers 100 forming the directional speaker 109 is structured in such a way that electrodes 102 are fixed on a base 101 and, at the tips of the electrodes 102, a diaphragm 104 is pasted using an insulating adhesive 103. In addition, a piezoelectric element 105 is pasted on the diaphragm 104 as a vibration generator. In some cases, a resonator 106 is pasted on the piezoelectric element 105 in order to increase the sound pressure of emitted sound. Furthermore, the piezoelectric element 105 is connected to the electrodes 102 via lead wires 107 so that the piezoelectric element 105 can be vibrated by signals sent from an external electric circuit (not shown).
The directional speaker described above, which generates secondary sound waves from ultrasonic waves through the parametric effect, has the problem that the efficiency of conversion from ultrasonic waves to audible sounds during audible sound reproduction is low. This makes it difficult to reproduce audible sounds using one ultrasonic speaker 100 and, as a result, many ultrasonic speakers 100 must be arranged in an array form as shown in FIG. 23. Because the speaker device becomes large, it becomes difficult to mount a directional speaker, configured in an array form, on a small electronic device or a portable terminal.
In addition to the directional speaker configured in an array form described above, a directional speaker using ultrasonic waves as carrier waves is also proposed. One of such directional speakers has a configuration in which ultrasonic carrier waves are amplitude modulated with sound signals and the resulting modulated signals are output from the ultrasonic resonator as a sound (for example, see Patent Documents 2 and 3).
The directional speaker using amplitude modulation described above has a problem that a sound with a high sound pressure cannot be generated. To solve this problem, the configuration that increases the output, for example, the configuration that increases the gain of the amplifier, is required.
Another directional speaker, which uses the ultrasonic wave as the carrier wave, is also proposed. This speaker frequency modulates the carrier wave, which is the ultrasonic wave, with the sound signal and outputs the resulting modulated signal from the ultrasonic resonator as a sound (for example, see Patent Document 4).
FIG. 25 is a block diagram showing the configuration of a directional speaker that uses frequency modulation. This directional speaker comprises sound generating means 110, ultrasonic wave generating means 120 for generating ultrasonic carrier waves, frequency modulation means 130 for frequency modulating ultrasonic waves generated by the ultrasonic generating means 120 with a sound signal generated by the sound generating means 110, amplifying means 140 for amplifying the modulated signal modulated by the frequency modulation means 130, and electric sound conversion means 150 for converting a modulated signal to a sound signal.
The above-described directional speaker described in Patent Document 4 generates a sound vibration in which the ultrasonic wave and the audible signal, emitted from the electric sound conversion means 150, are mixed as described in the document. As this sound vibration propagates through air as an ultrasonic wave, non-linear interaction occurs and the sound vibration is demodulated to an ultrasonic sound composed of low-frequency components.
[Patent Document 1]
Japanese Patent Laid-Open Publication No. 2003-47085 (FIGS. 1-2 in page 3)
[Patent Document 2]
Japanese Patent Laid-Open Publication No. Hei 3-159400
[Patent Document 3]
Japanese Patent Laid-Open Publication No. Hei 3-296399
[Patent Document 4]
Japanese Patent Laid-Open Publication No. Hei 11-164384
The inventor of this application has found that the audible sound obtained from a speaker with a configuration in which frequency modulation is used, such as the conventional directional speaker described in Patent Document 4 described above, is lower in the sound quality than that of the target sound signal to be output. This is because the sound pressure of an audible sound obtained by driving a frequency modulated wave with an ultrasonic speaker differs from the sound pressure of the target sound to be output.
FIG. 26 is a diagram showing how the sound pressure of an audible sound, obtained from a conventional frequency-modulation-based directional speaker, changes.
FIG. 26A shows the sound pressure distribution of a target sound to be output. A listener recognizes the sound pressure distribution, composed of a repetition of the high sound pressure part a and the low sound pressure part b, as a sound. FIG. 26B shows the sound signal of this sound. In this figure, the sound signal is represented by a sign wave signal at a predetermined frequency.
Frequency modulating the ultrasonic carrier wave shown in FIG. 26C with the sound signal shown in FIG. 26B gives the frequency modulated wave shown in FIG. 26D. Driving the diaphragm with this frequency modulated wave gives the audible sound with the sound pressure distribution shown in FIG. 26E.
Comparison between the sound pressure distribution of the target sound shown in FIG. 26A with the sound pressure distribution obtained from the frequency modulation shown in FIG. 26E indicates that the sound pressure distributions are different. A listener, who listens to the audible sound obtained from this frequency modulation, feels that the sound quality is degraded because of a change in the sound pressure distribution.
FIG. 27 shows a case in which the sound pressure of the target sound to be output is varied. Because the sound signal is at a fixed frequency in FIG. 26, the difference between the sound pressure distribution of the target sound shown in FIG. 26A and the sound pressure distribution obtained from the frequency modulation shown in FIG. 26E only appears to be a shift in phase. On the other hand, comparison between the sound pressure distribution of the target sound shown in FIG. 27A with the sound pressure distribution obtained from the frequency modulation shown in FIG. 27E, which is similar to the comparison in FIG. 26 described above, indicates more apparently that the sound pressure distributions are different.
As described above, the conventional directional speaker that uses a parametric effect has a problem that the speaker becomes large. The conventional directional speaker that amplitude modulates an ultrasonic carrier wave has a problem that it is difficult to obtain a high sound pressure.
The conventional directional speaker that frequency modulates an ultrasonic carrier wave has a problem that it is difficult to produce a good quality sound.